Acoustic pulse reflectometry is a non-invasive technique for determining the internal dimensions of a duct of variable cross-section. A generated acoustic pulse propagates down a reflectometer wavetube into the cavity of interest. The pressure amplitudes of the acoustic reflections are analyzed, so as to generate the input impulse response, which in turn allows the calculation of the cross-sectional area as a function of axial distance. The resulting area-distance curve, which consists of an equivalent acoustic area of the cavity versus axial length down the cavity, serves as a “one-dimensional image” of the cavity of interest.
In medical context, this capability may be used to distinguish signals arising from body cavities, such as the trachea and the esophagus because the characteristic area-distance profile are quite different from these two structures. For example, for a human endotracheal tube (ETT) airway cavity, the profile shows a constant cross-sectional area throughout the length of the ETT followed by an increase in the cross-sectional area corresponding to the increase of area of more distal part of the lung. During human esophageal intubation, the profile shows a constant cross-sectional area throughout the length of the ETT, followed by a sudden decrease in the cross-sectional area. This occurs because the nonrigid human esophagus is soft and collapses around the distal end of the ETT, thereby preventing further transmission of the acoustic impulse down the cavity.
Because acoustic reflectometry is based on physical principles, it does not rely on detection of carbon dioxide to distinguish between an esophageal and an endotracheal intubation. This is vital in the cardiopulmonary arrest setting when capnography may be useless because the patient has little or no pulmonary circulation and, therefore, may not produce a detectable amount of exhaled carbon dioxide.
The current art, Hood Labs reflectometer (Pembroke, Mass.), shown in FIG. 11, uses an internal wavetube, which is connected to a duct that is to be explored. When repetitive impulses are emitted from the reflectometer, the impulses emerge from the wavetube, traverse through the duct, and are reflected back toward the wavetube, where they are reflected further still within the wavetube. The Hood Labs time-domain acoustic pulse pressure reflectometer uses a series of equally spaced impulses of 0.2 ms duration. It uses the well-known Gopillaud-Ware-Aki (GWA) algorithm to keep track of the course of the emitted impulses and of their reflections, whether reflected from the duct (single and higher order internal duct reflections) or internally reflected within the wavetube.
Currently available acoustic reflectometers (ARs) are generally of two types: single-microphone AR and two-microphone AR. The major limitation of the single-microphone GWA algorithm-based AR is its unwieldiness, which is due to the long wavetube required to separate the duct reflection from the unwanted source reflection. Such a bulky single-microphone AR system is essentially unusable in the limited space confines offered by an operating room or an ICU.
For the two-microphone AR, the initialization of such a system is integrated with the GWA algorithm which demands that the inherent instability in the system be addressed by making the first non-zero pressure to be larger than some minimal value. Although a correction procedure may be used to correct the error introduced by this threshold, as the distance between the two microphones is reduced, the instability of the algorithm is inevitably increased. The inherent instability of the two-microphone algorithm significantly limits miniaturization of such a device.
Therefore, there still exists a need for a better approach to the design of AR that is simple, small, portable, fast and reliable such that the device may be practically employed in locales with minimal work space. Current art devices cannot be used for a prolonged period of time in a patient humidified breathing circuit, and require detachment of the breathing tube in order to obtain an area-distance profile.